This invention relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system with a painless shocking lead impedance measurement circuit.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias is via drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, or pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire having one or more electrodes disposed in the heart. Heart contractions are initiated in response to such pace pulses. By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem that arises in cardiac rhythm management devices is in determining defibrillation or xe2x80x9cshockingxe2x80x9d lead impedance. The defibrillation lead impedance includes the effective resistance of the leadwire that couples the cardiac rhythm management device to the heart for delivering the electrical defibrillation countershock at defibrillation electrodes located at or near the heart. The defibrillation lead impedance also includes the effective resistance of the body tissue (e.g., the heart) and body fluids located between the defibrillation electrodes. The defibrillation lead impedance due to the leadwire and heart resistance is generally around 50 xcexa9, but can range from 15 xcexa9 to 100 xcexa9.
The value of the defibrillation lead impedance provides useful information. For example, an extremely low lead impedance value may indicate a short circuit between the defibrillation electrodes. An extremely large lead impedance value may indicate an open circuit such as, for example, resulting from a leadwire that has become disconnected from the cardiac rhythm management device. Both defective leadwire conditions must be detected and remedied if the cardiac rhythm management device is to provide effective defibrillation countershock therapy to the heart.
It is possible to calculate defibrillation lead impedance, for example, by delivering an electrical defibrillation countershock to the heart. By measuring a voltage droop of the defibrillation countershock voltage pulse, the effective resistance of the defibrillation lead can be estimated. However, delivering a defibrillation countershock is painful to the patient. As a result, such techniques cannot be performed routinely, periodically, or even occasionally, because measuring defibrillation lead impedance because significant patient discomfort would likely result. Moreover, measuring defibrillation lead impedance is particularly difficult because of the high voltages (e.g., 750 Volts) being delivered to the heart during the defibrillation countershock. Typical measurement integrated circuits are not capable of withstanding such high voltages. In summary, there exists a need for a technique of measuring defibrillation lead impedance without inflicting pain on the patient from delivering a defibrillation countershock to measure defibrillation lead impedance.
The present cardiac rhythm management system provides, among other things, a defibrillation lead impedance measurement system. Instead of measuring defibrillation lead impedance by delivering a high energy or low energy defibrillation countershock and measuring a resulting voltage, defibrillation lead impedance is measured using a test current source that is different from the defibrillation output supply. A voltage resulting from the test current flowing through the defibrillation lead and heart resistance is measured. The defibrillation lead impedance is determined from the measured voltage.
Because low amplitude test current pulses are used (e.g., 10-20 milliamperes), the defibrillation lead impedance measurement would not cause significant pain or discomfort to the patient. As a result, the defibrillation lead impedance can be measured routinely for diagnostic or other purposes.
In one embodiment, the test currents are charge-balanced, i.e., a first test current pulse waveform sourced at a particular defibrillation electrode is offset by a substantially equal amount of charge sunk at that defibrillation electrode by a second test current pulse waveform, because the first and second test currents flow in opposite directions. This avoids charge build-up in the heart that may increase the difficulty of sensing intrinsic electrical heart activity signals. It also avoids degeneration of the defibrillation electrodes by electroplating or corrosion.
One embodiment provides test current pulses of at least two different steady-state amplitude steps. Defibrillation lead impedance is determined differentially by measuring a voltage associated with each test current amplitude step, and dividing a difference of the measured voltage by a corresponding difference in the test current amplitudes. This technique allows cancellation of a component of the measured voltage that is not associated with the desired defibrillation lead impedance measurement.
Another embodiment provides bidirectional test currents to account for polarity effects on the defibrillation lead impedance measurement. A further embodiment provides a calibration/correction technique in which measurements of known resistances are used to correct a measurement of an unknown defibrillation lead impedance measurement. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.